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Radar Speed Detector 

Make] Projects 

Radar Speed Detector 

Written By: Ken Delahoussaye 


Diagonal cutters (1) 
Drill d) 
File (1) 

Hacksaw (1) 
Multimeter d) 
Phillips screwdriver (1) 
Soldering iron (1) 
Wire strippers (1) 


Hot Wheels Radar Gun d) 

Mattel #J2358 

Project enclosure (1) 

Alkaline batterv (4) 

Rocker switch (1) 

Shielded cabled) 

Steel wired) 

DIN 5-pin male plug d) 

DIN 5-pin female receptacle d) 

Steel bard) 
or aluminum 
PVC document tube d ) 
sue Tube 
PVC threaded nipple d) 
Shrub sprinkler head d) 
• Bolts (2) 

Sheet metal screws (4) 

Machine screw (2) 
to fit DIN jack 
Camera tripod d) 
Double-sided tape d) 


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Radar Speed Detector 

I was browsing through a department store one day, in search of a gift for my 8-year-old 
daughter, when I came across Mattel's Hot Wheels Radar Gun ($30). The box said that this 
toy could clock the speeds of not only miniature Hot Wheels cars, but also full-sized 

I figured the toy must have severe limitations, but decided to buy one for my daughter 
anyway. It turns out that she (we) loved it, and we found that it could accurately measure the 
speeds of toy cars, cars on the road, even joggers. To my amazement, the detector even 
measured the speeds of spinning objects like bicycle wheels. 

Operating the toy is simple: you aim the gun, squeeze the trigger, and then read the detected 
speed on the LCD in back. Hold the trigger down for a while and then release, and you'll get 
the maximum speed detected during that time. A switch selects either mph or kph readings, 
and another switch toggles the display units between 1 :64 scale (for Hot Wheels) and 1 :1 
scale actual speeds. Power comes from 4 AAA batteries housed in the handle. 

Inside, the Mattel gun uses Doppler radar, just like the expensive speed detection systems 
used by law enforcement. It transmits a continuous wave at 10.525GHz, then measures the 
frequency that returns after the wave bounces off a moving target. The main functional 
difference between the Mattel toy and a $1 ,000-plus pro model is detection range, which for 
the toy maxes out at 40 feet. I suspect that this keeps the microwave emissions low enough 
to guarantee child safety. 

Limitations aside, I realized that this so-called toy offers some interesting prospects. The 
wheels in my mind began to churn, and I decided to purchase another unit for my own use. I 
disassembled the gun and decided to repackage it to appear more professional — looks 
really are everything. 

I separated the detector component itself (the waveguide antenna) from the display and 
control panel, then connected the two with a length of instrumentation cable. This 
configuration lets you position the antenna close to traffic on a tripod, and operate it remotely 
from a safe distance. 

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Radar Speed Detector 

Step 1 — Disassemble the toy. 

• Disassembly is no easy task, as 
there are 12 screws, each 
concealed with plastic inserts. I 
used a drill and W bit to carefully 
drill out the inserts and gain access 
to the screws. Use extreme caution 
with the drill, since some of the 
screws are located very close to 
sensitive internal components. 

• After removing all the screws and 
opening the case, you'll see the 
long, cylindrical waveguide antenna 
and a small circuit board that 
carries the buttons and LCD panel. 
(The waveguide is a hollow tube 
that surrounds the microwave 
antenna and directs and 
concentrates its signal.) 

• After recording their locations so 
you can reattach them correctly 
later, unsolder all the wires that 
connect the waveguide, battery 
compartment, and trigger switch. 
Be careful when working with the 
waveguide, which is made of a thin, 
dielectric material and dents easily. 
After detaching all the wires, 
remove the waveguide and display 
panel, and set them aside. 

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Radar Speed Detector 

Step 2 — Part I - Upgrade the antenna housing. 

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• For the waveguide antenna housing, I chose an "Ice Tube" by Alvin. I've used these 3" 
diameter acrylic document tubes in previous projects, and I like them. First and foremost, 
they look cool and futuristic — transparent with various tints. They're also fairly rigid and 
you can easily cut them with a hacksaw. I used transparent green tubing and cut an &A" 

• For a post that holds up the housing and makes it attachable to a tripod, use a 3 A" diameter 
threaded PVC nipple, 8" in length. Drill a W hole in one end of the acrylic tube and insert 
the nipple through it. Then screw a shrub sprinkler head onto the end of the nipple. Just 
below the sprinkler head, drill a Vi" exit hole to route the cable through. 

• Secure the sprinkler head to the wall of the tube opposite the hole, using the screw that 
came with the sprinkler head plus 2 smaller screws on either side to prevent rotation. For 
these, drill 3/32" holes through the tube and head, and apply two 1"x 1 /4" sheet metal 

• To secure the waveguide centered inside the tube, use 12-gauge steel wire, carefully 
wrapped around the waveguide in a spring-like fashion to form 2 mounts, one at each end. 
Pressure and friction on this wire are what holds the waveguide in the tube housing. There 
are no screw attachments. 

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Radar Speed Detector 

Step 3 — Part II - Upgrade the antenna housing. 


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• Before inserting the waveguide with mounts into the housing, solder on a new, longer 
interface cable that will support remote operation. The toy originally used a 4" length of 
shielded 2-conductor cable, so I figured that the new cable should also be shielded. 

• In my garage, I found a 20' length of instrumentation cable with shielding. Cut and strip one 
end of the cable, and solder 2 of the available 4 signal wires to the signal contacts on the 
antenna's round printed circuit board. Also solder the uninsulated drain wire, which 
connects to the shielding, to the board's ground contact in the middle. 

• Route the opposite end of the cable through the waveguide housing and PVC nipple, and 
then carefully push the waveguide assembly into the housing. To attach the housing to a 
camera tripod, remove the tripod's head assembly and support tube from the base, then 
route the antenna cabling through the hole, and slide in the PVC nipple and waveguide 

• The other end of the cable would attach to the display, but I wanted to make it easily 
detachable for transport. So I obtained a 5-pin DIN male connector plug and matching jack. 
Solder the 3 conductors that were connected at the other end to 3 contacts on the DIN 

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Radar Speed Detector 

Step 4 — Part I - Build the display housing. 

• To house the radar detector's display and controls, originally on the back of the toy gun, I 
used a 6"x3"x2" project case. First I made a carrying handle out of a bar of 0.1" thick steel 
(aluminum would have been easier to work with). 

# Drill Vi" holes in each end of the steel and bend it into a U shape. Drill corresponding holes 
in the display housing and attach the handle using hex bolts, nuts, and washers. One of 
the nice things about the handle is that it also functions as a stand, allowing hands-free 
viewing when the display unit is on a tabletop or other flat surface. 

• I needed to make a large hole in the lid of the display housing to fit the LCD display panel. 
Since I didn't have any large-diameter bits, I used a W bit to make a series of small 
perforations in the plastic. Punch out the section with diagonal cutters, and file the edges 
smooth. Then install the LCD panel to the lid by running a couple of M8"x 3 A" sheet metal 
screws through its 2 original mounting holes. 

# To replace the functionality of the original momentary trigger switch, I chose a double-pole, 
double-throw rocker switch. This has the advantage of allowing for automatic, hands-free 
speed measurements. To install the switch, drill a 3 A" hole into the display housing lid, 
centered below the display, and secure the switch in place with its retaining nut. To 
reinforce the display and switch, you might also add some glue. 

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Radar Speed Detector 

Step 5 — Part II - Build the display housing. 

• On the top of the display housing, install the female DIN connector receptacle, for plugging 
in the cable. Drill a 3 A" hole into the top of the display housing and attach the jack on the 
inside using a pair of 3mmx8mm screws and nuts. 

• The original radar gun used 4 AAA batteries. This would be fine for our new configuration, 
but since there is plenty of room inside the display housing, I decided to go with AAs, 
which have twice the capacity of their AAA cousins. 

• A quick measurement with my multimeter revealed that the radar system draws about 
24mA from the batteries connected in series. This translates to almost 100 hours of 
continuous use with AA alkalines (assuming 2200mAh each). Install a 4-AA battery holder 
inside the display housing using double-stick mounting tape. 

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Radar Speed Detector 

Step 6 — Solder all the connections. 



• With all the hardware installed inside the display housing, it's time to solder all the 
connections. The wires connect to 3 components: power switch, batteries, and the DIN 
connector for the antenna. For the DIN jack, the toy's original 4" antenna cable had 3 
wires, red, white, and a shielding conductor covered with black heatshrink. Solder these 3 
wires to the connector contacts matching the other end of the cable, using a multimeter to 
make sure you get the correspondence right. 

• The toy's power switch wiring consists of 4 individual wires: 2 brown and 2 blue. Position 
and solder these to the DPDT switch, following the same pattern as the original manual 
trigger switch. 

• Finally, the toy's battery wiring consists of 2 individual wires, red and black. Solder these 
directly to the corresponding contacts on the battery holder. That's it for the wiring. After 
closing up the case and plugging in the antenna cable, you're ready to go. 

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Radar Speed Detector 

Step 7 — Let the fun begin! 

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Radar Speed Detector 

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• I started out by having a family 
member walk in front of the 
antenna unit. At a normal pace, the 
display registered 2 mph, and a 
more brisk pace yielded a reading 
of 4-5 mph. 

• Next, I decided to try measuring 
the speed of a spinning bicycle 
wheel. The tripod was handy for 
this test, since it let me focus the 
waveguide precisely where I 
wanted. I placed the bicycle upside 
down on the floor, supported by its 
handlebars and seat. Then I turned 
the pedal to get the rear wheel 
spinning, and I clocked it at a 
maximum rate of 15 mph. So far, 
so good. 

• Then it was time for the real test, 
with actual cars on the road. I took 
the unit outside near the street and 
set up the tripod. It wasn't long 
before a vehicle came along, and 
the readout showed 19 mph. That 
was great — the detector was 
showing a speed for a passing 
automobile. But I wondered about 
its accuracy, so I decided to get 
into my car and drive down my 
street past the detector myself. 
Watching my speedometer, I drove 
at a steady speed of 21 mph. 
Heading back to my driveway, I 
was anxious to see what the device 
had measured, and to my delight 
the reading was 21 mph! 

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Radar Speed Detector 

• I've done several projects over the 
years, and this one has been one 
of the most satisfying. The Hot 
Wheels Radar Gun has some great 
hardware inside, and for my 
minimal investment, I now have a 
system that provides a useful 
function that I'm sure I'll be using 
and enjoying quite a bit for years to 

This project first appeared in MAKE Volume 10 . page 148. 

This document was last generated on 2013-01-15 02:24:10 PM. 

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